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Akatsuki(あかつき, 暁?, "Dawn"), also known as the Venus Climate Orbiter (VCO) and Planet-C, is a Japanese (JAXA) space probe tasked to study the atmosphere of Venus. It was launched aboard an H-IIA 202 rocket on 20 May 2010,[6] and failed to enter orbit around Venus on 6 December 2010. After the craft orbited the Sun for five years, engineers placed it into an alternative elliptical Venusian orbit on 7 December 2015 by firing its attitude control thrusters for 20 minutes.[4][5][7][8] By using five different cameras, Akatsuki will study the stratification of the atmosphere, atmospheric dynamics, and cloud physics.[9][10]

Contents

Mission

Akatsuki is a Japanese space mission to the planet Venus. Planned observations include cloud and surface imaging from an orbit around the planet with an infrared camera, which are aimed at investigation of the complex Venusian meteorology. Other experiments are designed to confirm the presence of lightning and to determine whether volcanism occurs currently on Venus.[11] In most planets, the atmosphere circulates much slower than the rotation speed of the planet. However, on Venus, while the planet rotates at 6 km/h at the equator, the atmosphere spins around the planet at 300 km/h.

Akatsuki is Japan's first planetary exploration mission since the failed Mars orbiter Nozomi probe which was launched in 1998. Akatsuki was originally intended to conduct scientific research for two or more years from an elliptical orbit around Venus ranging from 300 to 80,000 km (190 to 49,710 mi) in altitude,[1] but its alternate orbit, yet to be characterized, had to be highly elliptical. The budget for this mission is ¥14.6 billion (US$174 million) for the satellite and ¥9.8 billion (US$116 million) for the launch.[12]

Spacecraft design

The main bus is a 1.45 × 1.04 × 1.44 m (4.8 × 3.4 × 4.7 ft) box with two solar arrays, each with an area of about 1.4 m2 (15 sq ft). The solar arrays provide over 700 W of power in Venus orbit. The total mass of the spacecraft at launch was 517.6 kg (1,141 lb).[1] The mass of the science payload is 34 kg (75 lb).[13]

Propulsion is provided by a 500-newton (110 lbf) bi-propellant, hydrazine-dinitrogen tetroxide orbital maneuvering engine and twelve mono-propellant hydrazine reaction control thrusters, eight with 23 N (5.2 lbf) of thrust and four with 3 N (0.67 lbf). It is the first spacecraft to use a ceramic (silicon nitride) retrofire thruster. The total propellant mass at launch was 196.3 kg (433 lb).[1]

Communication is via an 8 GHz, 20-watt X-band transponder using the 1.6 m (5 ft 3 in) high-gain antenna. The high-gain antenna is flat to prevent heat from building up in it.[10] Akatsuki also has a pair of medium-gain horn antennas mounted on turntables and two low-gain antennas for command uplink. The medium-gain horn antennas are used for housekeeping data downlink when the high-gain antenna is not facing Earth.[1]

Instruments

The scientific payload consists of six instruments. The five imaging cameras will explore Venus in wavelengths from ultraviolet to the mid-infrared:[14][15]

the Lightning and Airglow Camera (LAC) will look for lightning in the visible wavelengths of 552 to 777 nanometers

the ultraviolet imager (UVI) will study the distribution of specific atmospheric gases such as sulfur dioxide in ultraviolet wavelengths (293–365 nm)

the longwave infrared camera (LIR) will study the structure of high-altitude clouds at a wavelength where they emit heat (10 μm)

the 1 μm camera (IR1) will image heat radiation emitted from Venus's surface rocks (0.90–1.01 μm) and will help researchers to spot active volcanoes, if they exist

the 2 μm camera (IR2) will detect heat radiation emitted from the lower reaches of the atmosphere (1.65–2.32 μm)

the Ultra-Stable Oscillator (USO) for high precision measurement of distance and communication

Public relations

A public relations campaign was held between October 2009 and January 2010 by the Planetary Society and JAXA, to allow individuals to send their name and a message aboard Akatsuki.[16][17] Names and messages were printed in fine letters on an aluminium plate and placed aboard Akatsuki.[16] 260,214 people submitted names and messages for the mission.[18] Around 90 aluminum plates were created for the spacecraft,[19] including three aluminium plates in which the images of the VocaloidHatsune Miku and her super deformed figure Hachune Miku were printed.[20]

Operations

Launch

The launch of Akatsuki

Akatsuki left the Sagamihara Campus on 17 March 2010, and arrived at the Tanegashima Space Center's Spacecraft Test and Assembly Building 2 on 19 March. On 4 May, Akatsuki was encapsulated inside the large payload fairing of the H-IIA rocket that launched the spacecraft, along with the IKAROSsolar sail, on a 6-month journey to Venus. On 9 May, the payload fairing was transported to the Tanegashima Space Center's Vehicle Assembly Building, where the fairing was mated to the H-IIA launch vehicle itself.[21]

The spacecraft was launched on 20 May 2010 at 21:58:22 (UTC) from the Tanegashima Space Center,[11] after being delayed because of weather from its initial 18 May scheduled target.[22]

Orbit insertion failure

Akatsuki was planned to initiate orbit insertion operations by igniting the orbital maneuvering engine at 23:49:00 on 6 December 2010 UTC.[21] The burn was supposed to continue for twelve minutes, to an initial orbit of 180,000 to 200,000 km (110,000 to 120,000 mi) apoapsis / 550 km (340 mi) periapsis / four days orbital period around Venus.[23]

The orbit insertion maneuver was confirmed to have started on time, but after the expected blackout due to occultation by Venus, the communication with the probe did not recover as planned. The probe was found to be in safe-hold mode, spin-stabilized state with ten minutes per rotation.[24] Due to the low communication speed through the low-gain antenna, it took a while to determine the state of the probe.[25] JAXA stated on 8 December that the probe's orbital insertion maneuver had failed.[26][27] At a press conference on 10 December, officials reported that Akatsuki's engines fired for less than three minutes, far short of what was required to enter into Venus orbit.[28] Further research found that the likely reason for the probe malfunction was salt deposits jamming the valve between the helium pressurization tank and the fuel tank. As a result, engine combustion became oxidizer-rich, with resulting high combustion temperatures damaging the combustion chamber throat and nozzle. A similar vapor leakage problem destroyed the NASAMars Observer probe in 1993.[29]

Recovery efforts

JAXA developed plans to attempt another orbital insertion burn when the probe returned to Venus in December 2015. This required placing the probe into "hibernation" or safe mode to prolong its life beyond the original 4.5-year design. JAXA expressed some confidence in keeping the probe operational, pointing to reduced battery wear, since the probe was then orbiting the Sun instead of its intended Venusian orbit.[30]

Telemetry data from the original failure suggested that the throat of its main engine, the orbit maneuver engine (OME) was still largely intact, and trial jet thrusts of the probe's onboard OME were performed twice, on 7 and 14 September 2011.[21] However, the thrust was only about 40 newtons (9.0 lbf), which was 10% of expectations. Following these tests, it was determined that insufficient specific impulse would be available for orbital maneuvering by the OME. It was concluded that the remaining combustion chamber throat was completely destroyed by transient ignition of the engine. As a result, the selected strategy was to use four hydrazine attitude control thrusters, also called reaction control system (RCS), to drive the probe into orbit around Venus. Because the RCS thrusters do not need oxidiser, the remaining 65 kg of oxidiser (MON) was vented overboard in October 2011 to lighten the spacecraft.[29]

Three peri-Venus orbital maneuvers were executed on 1 November,[11] 10 and 21 November 2011 using the RCS thrusters. A total delta-v of 243.8 m/s was imparted to the spacecraft. Because the RCS thrusters' specific impulse is low compared to the specific impulse of the OME, the previously planned insertion into low Venusian orbit became impossible. Instead, the new plan was to place the probe in a highly elliptical orbit with an apoapsis of a hundred thousand kilometers and a periapsis of a few thousand kilometers from Venus. Engineers planned for the alternate orbit to be prograde (in the direction of the atmospheric super-rotation) and lie in the orbital plane of Venus. The method and orbit were announced by JAXA in February 2015, with an orbit insertion date of 7 December 2015.[31] The probe reached its most distant point from Venus on 3 October 2013 and had been approaching the planet since then.[32]

Orbit insertion

After performing the last of a series of four trajectory correction maneuvers between 17 July and 11 September 2015, the probe was established on a rendezvous trajectory with Venus. The rendezvous occurred on 7 December 2015, when Akatsuki successfully entered Venus orbit after a 20-minute burn with 4 thrusters that were not rated for such a hefty propulsive maneuver.[4][5][33] Instead of taking about 30 hours to complete an orbit around Venus—as was originally planned—Akatsuki will complete one orbit every nine days after an adjustment in March 2016.[3]

Status

After JAXA engineers measured and calculated its orbit following the December 7 operation, JAXA announced on December 9 that Akatsuki had successfully entered the intended elliptical orbit, as far as 440,000 km (270,000 mi) from Venus, and as close as 400 km (250 mi) from Venus's surface with an orbital period of 13 days and 14 hours.[34] A follow-up thruster burn scheduled for 26 March 2016, will lower Akatsuki's peak altitude of its orbit to about 330,000 km (210,000 mi) and shorten its orbital period from 13 to 9 days.[3] The 2-year science phase will start after the March 2016 orbit adjustment.[3]

Having flown as close as 0.6 AU from the Sun, the gradual deterioration of heat insulation blankets was noticed, but the deterioration rate slowed in 2015 and the spacecraft survived the orbit transfer.[35] It was unknown whether the cameras and related electronics aboard sustained damage, as temperatures within the spacecraft rose 30 to 40 °C above design parameters.[29] Two days after orbit injection, controllers had verified the function of three of its cameras (UVI, LIR, IR1) and were planning verification of the other 3 scientific instruments (IR2, LAC, USO) in the next few days.[36]